1. Field of the Invention
The present invention relates to a semiconductor apparatus including an antenna switch for a wireless communication system. In particular, the present invention relates to an effective technology to significantly reduce or prevent deterioration in characteristics caused by temperature fluctuation in the antenna switch.
2. Description of the Related Art
A typical mobile terminal, such as a cellular phone, for wireless communication uses an antenna switch for switching a signal path connected to one antenna between the transmission side and the reception side. In the antenna switch used in the mobile terminal, a high-frequency switch in an on state is required to have low insertion loss while a high-frequency switch in an off state is required to have high isolation characteristics and low distortion characteristics. Many antenna switches using high electron mobility transistors (HEMTs) that have a hetero junction structure and that have low on resistance as switch elements have been put into production in recent years as antenna switches having low insertion loss. A technology to improve the isolation characteristics of a field effect transistor in the off state in an antenna switch using the field effect transistor, such as the HEMI, is disclosed in Japanese Unexamined Patent Application Publication No. 2010-114837.
Japanese Unexamined Patent Application Publication No. 2010-114837 discloses an antenna switch in which, in transmission of a radio-frequency (RF) transmission signal, a voltage step-up circuit generates a positive voltage higher than direct current (DC) control voltage on the basis of the DC control voltage and the RF transmission signal and the generated positive voltage is used as a gate control voltage of a transmission-side field effect transistor (an n channel HEMT). With this antenna switch, the on resistance of the transmission-side field effect transistor in the transmission is decreased to reduce the insertion loss, and the voltage at the source side of a reception-side field effect transistor is increased via the gate of the transmission-side field effect transistor to strengthen a reverse bias state of the reception-side field effect transistor in order to improve the isolation characteristics.
In addition, Japanese Unexamined Patent Application Publication No. 2010-114837 discloses an antenna switch in which, in the transmission of the RF transmission signal, a voltage step-down circuit generates negative voltage lower than the DC control voltage on the basis of the DC control voltage and the RF transmission signal and the generated negative voltage is used as the gate control voltage of the reception-side field effect transistor (an n channel HEMT) to put a gate Schottky barrier of the reception-side field effect transistor in the reverse bias state. With this antenna switch, the reverse bias state of the reception-side field effect transistor is strengthened to improve the isolation characteristics, as in the voltage step-up circuit described above.
The voltage step-up circuit and the voltage step-down circuit charge capacitors by using switching of the polarity of the transmission signal, like a charge pump circuit, to generate the positive voltage higher than the DC control voltage and the negative voltage lower than the DC control voltage, respectively. In the circuit that generates the positive voltage or the negative voltage according to the above principle, the time to cause the output voltage to have a desired value is mainly determined by the charging time of the capacitor. Accordingly, for example, if the charging time of the capacitor in the voltage step-down circuit is long in the antenna switch including the voltage step-down circuit described in Japanese Unexamined Patent Application Publication No. 2010-114837, it takes a long time to put the reception-side field effect transistor in a sufficient off state (the reverse bias state) in the transmission of the RF transmission signal to form a leakage path of the RF transmission signal at the reception side, thereby possibly deteriorating the characteristics of harmonic distortion in the RF transmission.
The charging time of the capacitor is mainly determined by the time constant of the capacitor to be charged and the resistor existing on the charge path. For example, in the voltage step-down circuit illustrated in FIG. 1 in Japanese Unexamined Patent Application Publication No. 2010-114837, the charging time is mainly determined by the time constant of a capacitor 110 and a resistor 107. In order to shorten the charging time of the capacitor, it is necessary to decrease the resistance value of the resistor existing on the charge path as much as possible. However, since the resistor has a role in limiting the amount of current flowing through the capacitor so as not to exceed an allowable value, it is not possible to cause the resistor to have an extremely low resistance value. Accordingly, in designing the circuit, it is necessary to determine the resistance value so as to shorten the charging time while limiting the amount of current flowing through the capacitor so as not to exceed the allowable value. However, since the resistor formed on a semiconductor substrate has positive temperature characteristics or negative temperature characteristics, the resistance value is increased at low temperature or high temperature even if the resistance value is determined so that the charging time of the capacitor is set to an appropriate time at normal temperature to increase the charging time. For example, provided that the gate voltage of the reception-side field effect transistor is made, for example, −2.0 V when a certain time elapsed after the transmission signal rises at normal temperature, the gate voltage of the reception-side field effect transistor is only made −1.0 V at high temperature even if the same time elapsed and it takes a longer time to make the gate voltage of the reception-side field effect transistor −2.0 V. Accordingly, even when no problem occurs at normal temperature, the leakage path of the RF transmission signal may be formed at the reception side immediately after the RF transmission signal rises to possibly cause the harmonic distortion of the RF transmission signal at high temperature or low temperature.
The communication quality of the mobile terminals is required to be further improved in recent years and the antenna switches used in the mobile terminals are also required to realize high specifications. Accordingly, the inventor has considered that it is not possible to ignore deterioration in characteristics caused by temperature fluctuation in the antenna switches.